S-adenosyl-L-methionine decarboxylase, a key enzyme in polyamine biosynthesis, has been purified to apparent homogeneity from mouse mammary gland and liver. By use of the purified enzyme, a monospecific mouse antibody to the enzyme has been prepared. The antibody was used in measuring levels of the enzyme during hormone-dependent development of the mammary gland. Alpha-lactalbumin, a specific milk protein, from mouse milk has been purified to apparent homogeneity, and its antibody has been prepared. The antibody has been used to study the hormonal regulation of milk protein synthesis. By use of immunochemical assay for milk proteins, a striking difference has been found between the optimal concentrations of cortisol required for maximal induction of casein and alpha-lactalbumin: 3 x 10 to the minus 8th power molar for alpha-lactalbumin and 3 x 10 to the minus 6th power molar for casein. Moreover, 10 to the minus 7th power to 10 to the minus 5th power molar cortisol caused progressive inhibition of alpha-lactalbumin. By the use of different hormone combinations and decreased concentration of cortisol, we have been able to identify cyclic AMP as a negative controlling factor for the induction of milk protein synthesis.